Laminar premixed flat propene and acetylene flames with different C/O ratios have been investigated at 50 mbar. Profiles of a variety of stable and radical species were obtained by molecular beam sampling mass spectrometry (MBMS). The temperature profiles were measured with laser-induced fluorescence (LIF) of OH and NO. In the high-temperature regime, the OH radical was probed, whereas NO was added to the fresh gases as a temperature indicator for the cooler regions. In evaluating the measurements, emphasis was laid on the formation of C-6 species, in particular of C6H6, as one of the key species in soot formation. Concentration profiles for more than 30 species have been analyzed, and the observed differences in the concentrations of several intermediates, in particular potential precursors of benzene, are discussed. A reaction how analysis has been performed to reveal that striking differences exist in the chemistry of fuel-rich propene and acetylene flames, a result which is of value for flame modeling. For the flames investigated, the observed differences in the concentration profiles of some potential precursors of benzene are discussed in comparison with the predictions of flame modeling. (C) 2000 by The Combustion Institute.